Vacuum deposition of coatings using cathode sputtering induced by glow discharges is currently in widespread use. Sputter coating sources include cathode and anode structures, and are operated in an evacuated chamber backfilled with a sputter gas (typically argon at subatmospheric pressure). Positive ions formed in the space between anode and cathode impact a target located on the cathode surface, ejecting (by sputtering) atoms of target material from the surface and near subsurface atomic layers of the target. These sputtered atoms deposit on workpieces or substrates placed generally in line of sight of the target. Magnetron sputter coating sources employ magnetic fields crossed with electric fields in the vicinity of the target. The use of such magnetic fields can enhance glow discharge intensities and the attendant sputtering rates, extend operation to lower sputter gas pressures, confine the glow discharge to the neighborhood of the electrodes, and reduce electron bombardment of the substrates.
One type of magnetron sputter coating source in commercial use employs a nonmagnetic annular sputter target (cathode) of a generally inverted conical configuration surrounding an axially symmetric central anode. One example of such a sputter coating source may be found described in U.S. Pat. No. 4,100,055, issued July 11, 1978 to Robert M. Rainey and entitled "Target Profile for Sputtering Apparatus" and assigned to the assignee of the present invention. Magnetron sputter coating sources of the type just mentioned have been used extensively and effectively in important semiconductor wafer coating applications. In most cases, the materials being deposited are nonmagnetic, such as aluminum and its alloys, etc. In some cases, however, it has been desired to use the same sputter coating source to dispense such magnetic materials as iron, nickel, iron-nickel-alloys, etc., as well as the nonmagnetic materials for which the sputter coating source was initially designed. More recently, a need has emerged for coating magnetic disk substrates with multiple layers, at least one of which is of magnetic material. Magnetic disks are now vitally important in computer memory applications.
Simply replacing a nonmagnetic sputter target with a magnetic one of the same generally inverted conical configuration in the magnetron sputter coating source referred to above causes most of the magnetic field to be shunted through the magnetic target. This results in magnetic field intensities above the target which are too low to allow the desired magnetic enhancement of the glow discharge to take place.
In order to avoid excessive reduction in magnetic field intensities above the target, annular magnetic sputter targets of a generally L-shaped profile have been developed for use in the above-described sputter coating source. One such L-shaped magnetic sputter target is described in U.S. Pat. No. 4,060,470, issued Nov. 29, 1977 to Peter J. Clarke and entitled "Sputtering Apparatus and Method". An essential feature of the L-shaped design is that the radial thickness of the outer annular band portion must be sufficiently thin that it is magnetically saturated in order that the magnetic field intensities above the target can be made sufficiently great that the desired magnetic enhancement of the glow discharge takes place. The higher the magnetic permeability and the saturation magnetization of the material, the thinner the annular band portion must be.
Magnetic sputter targets having an L-shaped profile contain much less material than nonmagnetic sputter targets of a generally inverted conical configuration. Moreover, the magnetic fields above the L-shaped magnetic targets lead to target erosion which is concentrated in the corner region. In relative terms, the inventory of magnetic target material usefully available for sputtering is therefore very limited.
It is also known that a magnetic material heated to or above its Curie temperature loses its ferromagnetism while so heated. Another approach to avoiding excessive reduction in magnetic field intensities above the sputter target, therefore, is to heat the target and maintain it at or above its Curie temperature. A disadvantage of this approach is that it requires means for monitoring the temperature of the target, coupled with a feedback system for achieving and maintaining the required Curie temperature. Also, the Curie temperature of some magnetic materials is so high as to be detrimental to the adjacent substrate being coated and/or to the vacuum seals for the system and/or to cause damage to the sputter coating source or target as a result of warping or excessive thermal expansion.
Most present-day magnetron sputter coating sources employ permanent magnets to provide the magnetic field required for glow discharge enhancement. As the sputter target erodes, the magnetic field intensities above the sputter target generally become stronger, leading to a lower electrical impedance of the glow discharge. This causes the sustainable voltage across the glow discharge to fall, bringing with it a decrease in sputtering yield. Maintaining a fixed sputtering rate, and hence a fixed coating rate, at a desired sputter gas pressure requires both higher current and a higher power. The glow discharge power supply must therefore be capable of providing extended ranges of voltage, current, and power, which adversely affects both power supply and power consumption costs.
Additional factors affect the electrical impedance of glow discharges. Sputter gas pressure is one. Others include thermal effects (expansion, contraction, and Curie-temperature-related) in sputter targets and magnetic circuits. The permanent magnet means used in most present-day magnetron sputter coating sources do not provide compensation of glow discharge impedance changes arising from such factors.
It is a well-known characteristic of glow discharges that the conditions for ignition (discharge initiation) and steady state operation are different. In some cases it is desirable to operate at a sputter gas pressure so low that ignition cannot occur with the magnetic fields in the sputter source (as provided by the usual permanent magnets) and the open-circuit voltage of the glow discharge power supply. One technique that can be used is to raise the sputter gas pressure sufficiently to allow ignition to occur, and then to reduce the sputter gas pressure to the desired operating level. Disadvantages of this approach include the relatively long time constants associated with the required pressure changes, plus the costs and complexity associated with controlling sufficiently quickly (that is, in times short in comparison with a coating cycle) the sputter gas pressure, which is normally controlled by flow rate and pumping speed.
Magnetic saturation of the cathode is the approach taken in the sputtering source described in U.S. Pat. No. 4,500,409 to Donald L. Boys et al issued Feb. 19, 1985, entilted "Magnetron Sputter Coating Source for Both Magnetic and Nonmagnetic Target Materials" and assigned to the assignee of the present invention. The magnetic field is provided by an electromagnetic coil and may be varied over a wide range of values. The coil surrounds an inner cylinder. A base plate connects the inner cylinder with an outer cylinder to form a magnetic yoke. A radial gap is provided between the inner and outer cylinders. The sputter target is positioned atop this gap. The yoke and the polepieces are made of ferromagnetic materials having high magnetic permeabilities and high saturation magnetizations, such as soft iron or a magnetic stainless steel. The base plate and cylinders have sufficiently large cross-sectional areas transverse to the direction of the internal magnetic flux lines that a very low reluctance (low magnetic resistance) path to the polepieces is provided at the maximum electromagnet coil current (to produce maximum magnetomotive force) required during operation.
When a sputter target of magnetic material is positioned atop the polepiece gap, it acts as a magnetic shunt at sufficiently low coil currents, whereby the magnetic field intensities adjacent the target are negligibly small. Upon increasing the coil current sufficiently, the onset of magnetic saturation of a portion of the target will occur. For a ring-shaped magnetic target of uniform thickness, this initial saturation will take place at a radius slightly greater than that of the inner polepieces, and fringing magnetic field lines will be established above and below the target near this radius. As the coil current is increased further, the magnetically saturated region will increase in radial extent, forming a high reluctance magnetic gap across which fringing magnetic field lines of increased intensity will be established. Unsaturated portions of the target will extend axially above the polepieces, and also radially inward and outward of the above-mentioned magnetic gap, thereby forming axial and radial extensions of the polepieces. At a particular required value of coil current, the configuration and intensities of magnetic fields above the target will permit the desired enhancement of the glow discharge to be achieved. This required value will increase with the magnetic permeability and the saturation magnetization of the target material, and also with target thickness. This source requires the thickness of the cathode be kept sufficiently thin to allow saturation and fringing. Power is limited by the low voltage and high currents needed to operate.
Concepts of the planar magnetron discharge closely parallel the common, magnetic-field free glow discharge between opposing electrodes at a pressure of one torr. Adjacent to the cathode, there is a cathode dark space, then a negative glow and then a positive column. The negative glow is an intense, ring-shaped discharge anchored to the cathode of the planar magnetron. An essential difference between the operation of the glow discharge sputtering source with or without a magnet is that the magnetic field allows for discharge to operate at a pressure of a few millitorr with minimal scattering and dissipation of the sputtered atoms by background gas.
In the deposition of magnetic materials, operation of the planar magnetron presents special problems. In the intense region of the discharge, the magnetic field is parallel to the surface of the cathode. Thus, if the cathode is a thick piece of magnetic material, the field may be shunted and the discharge will not operate. In two solutions to this problem, the cathode is driven beyond magnetic saturation or is slotted.
If a cylindrical shell anode is placed between two opposed cathodes and a magnetic field is imposed normal to the cathodes, the problem of shunting of the magnetic field is eliminated. The discharge is known as a Penning discharge. The enclosed geometry of the Penning structure is not suitable for thin film deposition on a planar substrate.